The present disclosure relates to an image forming apparatus, such as a copier, a printer, a facsimile machine, a multifunction peripheral having their functions integrated together, that is provided with a charging member in contact with an image carrying member. More particularly, the present disclosure relates to a method for estimating the life of a charging member.
In image forming apparatuses adopting an electro-photographic process, there are used, as a means for electrostatically charging uniformly the surface of a photosensitive drum which is an image carrying member, charging devices of a corona charging type such as scorotron charging devices and corotron charging devices provided with a corona discharge device, and charging devices of a contact charging type provided with an electrically conductive charging member as exemplified by a charging roller. In recent years, instead of the charging devices of a scorotron type or a corotron type, charging devices of a contact charging type are used that are provided with a charging member (such as a charging roller) which is arranged in contact with or close to a photosensitive drum and which electrostatically charges the photosensitive drum and that generate a smaller amount of ozone.
In the charging devices of a contact charging type, a charging member makes contact with a photosensitive drum, and thus toner external additive with high electrical resistance having slipped by a cleaning blade attaches to the charging member. As the attachment amount of toner external additive increases over a long period of use of the charging device, the chargeability of a part of the charging roller to which the toner external additive is attached decreases. This inconveniently makes the surface potential of the photosensitive drum locally low and results in a foggy image.
Conventionally, the toner external additive attached to the charging member is removed by bringing a charging member cleaning member comprising a sponge or a brush into contact with the charging member. However, performing continuous printing of high-density images or continuous printing in a high-temperature, high-humidity environment or in a low-temperature, low-humidity environment may increase the amount of toner external additive slipping by a cleaning blade and also increase the amount of toner external additive attaching to the charging member. Thus, with the charging member cleaning member, it is sometimes impossible to sufficiently remove the toner external additive attached to the charging member.
As a solution, there have been proposed methods for detecting the life of a charging member when its chargeability decreases due to the contamination of the charging member. For example, according to a known method, there are provided a charging means whereby the surface of an image carrying member is electrostatically charged uniformly by a charging member in contact with the surface of the image carrying member, an exposure means whereby the surface of the image carrying member is exposed to light, a detection means whereby the charging current passing during the electrostatically charging by the charging means is detected, and a means whereby a warning that the charging member reaches the end of its life is given when the charging current reaches a predetermined value, and, when the charging current for detection is equal to or lower than a set value, it is determined that the charging member has reached the end of its life and a warning is given.